Filling compound for and method of insulating conductors



April 1939- L. MCCULLOCH 2,154,276

FILLING COMPOUND FOR AND METHOD OF INSULATING CONDUCTORS Filed Feb. 17,1938 WITNESSES:

. INVENTOR Patented Apr. 11, 1939 PATENT OFFICE FILLING COMPOUND FOR ANDMETHOD OF INSULATING CONDUCTORS Leon McCulloch, Pittsburgh, Pa.,assignor to Westinghouse Electric & Manufacturing Company,

East Pittsburgh, Pa.,

sylvania a corporation of Penn- Application February 17, 1938, SerialNo. 191,037

8 Claims.

This invention relates to filling compounds for bushings and to theinsulating of the conductors from the weather casings of bushings.

In manufacturing bushings, the space between the conductor element andthe weather casing is filled with an insulating material, such as oil orthe like. In many applications, and in particular the bushings employedin conjunction with power transformers, oil is not altogethersatisfactory as a filling material because the high operatingtemperature which is encountered makes it quite difficult to maintain atight seal of the oil-filled bushing.

The gum compounds sometimes employed heretofore as the filling materialare not entirely satisfactory in that they have one or more defects.Some of the gum compounds employed have too high a dielectric powerfactor or have the defect that they are of so low density compared withwater that water may sink therein and be entrapped, Another defect ofcertain of these gum compounds is that they have a tendency to become sohard and brittle at temperatures below 0 F. as to contract and formvoids and cracks with the result that moisture can enter and corona mayoccur.

An object of this invention is the provision of a filling compoundsuitable for use in bushings.

Another object of this invention is the provision of a filling compoundfor use in bushings which is heavier than water and soft at lowtemperatures.

A further object of this invention is to protect electrical apparatusprovided with a bushing in which a conductor element is disposed inspaced relation to a weather casing by interposing a suitable fillingcompound between the conductor and the casing to increase the dielectricstrength of the path from the conductor to the casing.

Another object of this invention is to provide for insulating a casingfrom a conductor disposed in spaced relation therein.

Other objects of this invention will become apparent from the followingdescription when taken in conjunction with the accompanying drawing, thesingle figure of which is shown in elevation and partly in section,illustrates a transformer provided with a bushing that is protected inac cordance with the teachings of this invention.

Referring to the drawing, this invention is illustrated with referenceto a transformer l0 having a core l2 and coils l4 immersed in aninsulating and cooling medium l6 within a suitable casing 18. The casingI8 carries suitable lead-in bushings 20, which, in this instance, areillustrated as being of the condenser type, the conductors 22 orcondenser elements of which are spaced from the weather casing 24 ofporcelain or other suitable dielectric material and connected to thecoil assembly by suitable leads 26. As illustrated, a filling compound28 is employed in the bushing to fill the space between the conductorelements and the weather casing of the bushing.

In preparing the filling compound, it is desirable to employ a basematerial which may be melted and become fluid enough for pouring intothe space between the conductor elements and the weather casing. It isalso desirable that the base material be resilient and rubbery atordinary temperatures and have the property of being more or lesselastic at lower temperatures.

Asphalt, and particularly petroleum asphalt which is substantially freefrom mineral impurities, has these desirable properties and is preferredto be employed as the base material. A

preferred petroleum asphalt employed as the base material is one whichmay be purchased on the open market and has the properties of having aBall and Ring melting point of 115 C., a penetration of seconds at 25C., a pouring temperature of 175 C., a specific gravity of 1.02 at C.,and is less than 0.6% insoluble in benzol.

Since the petroleum asphalt has a density less than that of Water, inorder to increase the density to a value substantially above that ofwater, a chlorinated hydrocarbon is added to the asphalt. Thechlorinated hydrocarbon employed for this purpose is one of those knownin the open market as the aroclors which are derived from diphenyl andits derivatives and which has a chlorine content of between 55% and 70%and a preferred chlorine content of between 60% and 65%. The preferredaroclor is selected from that group of aroclors which arenon-crystalline,

. glassy, resin-like solids at room temperature and are soluble in oilin all proportions.

The non-crystalline aroclors are the chlorinated high boilingderivatives of diphenyl fractionated, the fractionating boiling rangebeing between 270 C. and 400 C. as prepared by the well known syntheticmanufacture of diphenyl by passing the vapors of benzene through moltenlead and subsequently fractionation of the product. These aroclors haveextremely high chemical stability, being resistant to oxidation and theeffects of moisture at moderately elevated temperatures withoutliberation of acid, and are of very low volatility. Further, they haveno appreciable swelling or softening action upon varnishes, and will,therefore, not effect the varnished paper-wound condenser elements whenemployed in the filling compound for condenser bushings.

In order that the filling compound may have the desired softness at lowtemperatures, a light oil is employed in conjunction with the asphaltand the aroclor. In order to prevent softening of the varnish on thecondenser elements of a condenser bushing, the oil employed must be ofanonaromatic nature. A preferred oil is the refined petroleum oil knownas transformer oil such as may be purchased on the open market havingexcellent electrical properties, low viscosity, low freezing point, andis stable, inert and non-volatile at ordinary temperatures.

In preparing the compound, it is desirable to mix the asphalt, aroclorand oil in such proportions that it will have a density greater thanthat of water. A compound having a density of 1.05 at 20 C. is found tobe satisfactory, since it floats water and still does not contain solarge a proportion of the aroclor as to prohibitively increase the costof the final compound. The following table lists a number of therepresentative compounds having the required properties and which aresatisfactory for use as a filling compound in the bushings.

Composition Serial No. Aroclor Asphalt Oil These compounds have aspecific gravity of about 1.05 at 20 C. and a pull away temperature ofbetween -40 F. and -85 F.

The pull away temperature of the filling compound is a measure of thehardness of the compound at low temperatures, and may be determined byplacing a quantity of the compound in a large glass tube which is thenstoppered and immersed in an alcohol bath. When immersed in the alcoholbath, the temperature of the bath is lowered in steps of 10 F. byemploying solid carbon dioxide and holding the bath at each particulartemperature for a period of time of about thirty minutes. When thetemperature of the bath is sufficiently lowered, the compound within theglass tube hardens and contracts enough to separate suddenly from thewalls of the glass tube emitting a. loud cracking sound. The temperatureat which this occurs is called the pull-away temperature. Where thepullaway temperature is -40 F. or lower, the compound is satisfactory asa filling compound for bushings.

The compounds prepared as hereinbefore described are quite viscous asmay be readily determined by following with X-rays the rise of large airbubbles through the compounds in twoinch glass tubes inclined at anangle of 22 and kept at a temperature of about 25 C. In testing thecompound identified h'ereinbefore as No. 183, it is found that thebubble of air travels 7 inches in 48 hours, while in compound No. 253,the bubble of air travels 8 inches in a corresponding period.

In these compounds, the non-crystalline aroclors stabilize the solutionof the asphalt in the mineral oil and render the compound stable at thesurface even after long exposure to water. The resistance of thecompound toward water is because of the high solvent power of thearoclor for the oil and the asphalt.

The compound described hereinbefore is sufficiently dielectric that,when employed as a filling compound in the bushing structure, itincreases the dielectric strength of the path from the conductorelements within the bushing to the casing and metal supporting flange ofthe bushing. Further, since the compound is heavier than water andresistant to moisture penetration as well as having the characteristicof being a substantially viscous gum down to temperatures of the orderof 40 F. or lower, the compound will function to exclude water ormoisture from the elements under substantially all conditions which maybe met in service. These compounds, because of their uniform quality andrelatively good chemical stability and the ease with which they may bemelted and poured or run into the space between a conductor element anda casing, may easily be interposed between the conductor element and thecasing of extremely slender bushing assemblies.

Although this invention has been described with reference to aparticular compound, it is, of course, not to be limited thereto exceptinsofar as is necessitated by the prior art and the scope of theappended claims.

I claim as my invention:

l. A filling compound for bushings comprising a petroleum asphalt, achlorinated hydrocarbon having the characteristic of being anoncrystalline, glassy, resin-like solid at room temperature, and apetroleum oil.

2. A filling compound for bushings comprising a petroleum asphalt, achlorinated hydrocarbon selected from the group of aroclors having achlorine content of between 55 and 70%, and a petroleum oil.

3. A filling compound for bushings comprising from 40 to 60% ofpetroleum asphalt, from 17 to 29% of aroclor having a chlorine contentof between 55 and 70%, and from 23 to 40% of potroleum oil.

4. A filling compound for bushings comprising about 58% of petroleumasphalt, about 17% of aroclor having a chlorine content of between 55and 70%, and about 25% of petroleum oil.

5. A filling compound for bushings comprising from 54 to 60% ofpetroleum asphalt, from about 17 to 18% of a chlorinated hydrocarbonselected from the group of aroclors having a chlorine content of between60 and 65%, and from about 23 to 28% of petroleum oil.

6. The method of protecting electrical apparatus provided with a bushingin which a conductor element is disposed in spaced relation to a weathercasing comprising interposing between the conductor and casing adielectric filling compound which comprises a petroleum asphalt, achlorinated hydrocarbon selected from the group of aroclors having achlorine content of between 55 and 70% and petroleum oil to increase thedielectric strength of the path from the conductor element to the casingand exclude moisture from the element.

7. The method of protecting electrical apparatus provided with a bushingin which a conductor element is disposed in spaced relation to weathercasing comprising interposing between the conductor and easing adielectric filling compound which comprises from 40 to 60% of petroleumasphalt, from 17 to 20% of a chlorinated hydrocarbon selected from thegroup of aroclors having a chlorine content of between 55 and 70% andfrom about 23 to 40% of petroleum oil, which compound increases thedielectric strength of the path from the conductor element to the casingand excludes moisture from the element.

8. The method of insulating a conductor from a casing which comprisesinterposing a dielectric filling compound between the conductor andcasing, the dielectric filling compound comprising from 54 to 60% ofpetroleum asphalt, from 17 to 18% of a chlorinated hydrocarbon selectedfrom the group of aroclors having a chlorine content of between about 55and 70% and from 23 to 28% 5 of petroleum oil.

LEON McCULLOCH.

